Glasses which are to be transformed into the form of fibers are produced from natural raw materials which contain various impurities, particularly iron oxide. When production conditions do not make it possible to obtain a sufficiently oxidized glass, serious drawbacks result. The ferrous iron/ferric iron ratio in the molten glass will increase and thereby cause a reduction of the conductivity of radiation of the glass. This reduction of the conductivity will be reflected by an increase in the difference in temperature between two given points in the molten glass bath. This phenomenon causes a change in the convection currents of the glass and will disturb the performance of the furnace, especially in the case of an electric furnace, which is more sensitive to the iron content than a draft furnace.
It is well known to introduce sodium sulfate, calcium sulfate or ammonium sulfate in the vitrifiable mixture, in order to facilitate melting of the mixture and aid in refining of the glass. When the glass is oxidized insufficiently, the dissolution capacity of the sulfates is lower. It follows that the melting rate of the mixture is slowed down, which can be compensated for only by an increase of the temperature in the melting zone. This requires an increase in the amount of energy used to melt the glass, and causes an accelerated wear of the refractories constituting the walls and the bottom of the furnace.
It is also necessary to emphasize that the reduction of the glass can cause its degassing. The excess of sulfate that it contains in the dissolved state is released in the form of SO.sub.2, which gives rise to an insulating layer between the vitrifiable mixture and the molten glass. This phenomenon considerably impedes the melting.
It is also known that the fibers obtained from an insufficiently oxidized glass exhibit diminished mechanical properties. This is clearly shown in "The effect of batch carbon on the strength of E-glass fibers", Glass Technology, p. 90-91, vol. 10 no. 3 (June 1969), with regard to E glass, which is universally used to produce continuous fibers.
Maintaining a minimum degree of oxidation is therefore imperative to assure proper operation of the glass production devices and the production of high quality fibers.
To prevent the drawbacks described above, it is customary to use raw materials containing a minimum of impurities, especially iron. These materials are extracted often far from the factories which use them, which proportionately increases their cost. It is also customary to prevent excessive amounts of products able to reduce the glass from being introduced into the vitrifiable mixture; this precaution limits in particular the amount of production waste, formed, for example, by fibers coated with organic products, able to be recycled. Until now, the only economical way to recycle a large amount of waste has been to increase considerably the sulfate level in the vitrifiable mixture, but such a process is environmentally unsound due to the air pollution it causes.